1. Field of the Invention
The present invention relates to a motor-driven power steering apparatus.
2. Description of the Related Art
In the motor-driven power steering apparatus, a pinion shaft connected to a steering shaft via a torsion bar is pivoted to a gear housing. A rack shaft engaging with the pinion shaft is supported to the gear housing so as to be linearly movable, and a worm gear connected to a drive shaft of an electric motor is pivoted to the gear housing. A worm wheel is fixed to an intermediate portion of the pinion shaft so as to be engaged with the worm gear is pivoted to the gear housing. The electric motor is structured such as to apply a steering assist torque in correspondence to a steering torque which a driver applies to the steering shaft to the rack shaft via an engagement between the worm gear and the worm wheel, and an engagement between the pinion shaft and the rack shaft.
In the motor-driven power steering apparatus mentioned above, it is necessary to easily set a center distance between the worm gear and the worm wheel without being affected by a dimensional error of a part such as a worm gear, at a time of assembling, and easily adjust the center distance in the case that the engagement between the worm gear and the worm wheel is changed with age after the assembling, thereby removing a backlash thereof.
In the motor-driven power steering apparatus described in Japanese Patent No. 3646205 (Patent document 1), it has a preload means which energizes a bearing supporting a leading end shaft portion of the worm gear in a predetermined preload direction in such a manner as to apply a preload to the engagement portion between the worm gear and the worm wheel. On the basis of an energizing force of the preload means, the center distance between the worm gear and the worm wheel is adjusted, and the backlash thereof is removed.
The preload means described in Japanese Patent No. 3646205 is structured such that a guide case accommodates a bearing and is provided with a guide surface guiding in such a manner that the bearing moves in the predetermined preload direction is fitted to a fitting hole of the gear housing. An adjusting screw is threadably attached to a retaining hole provided in the gear housing, and a coil spring backed up by the adjusting screw is inserted thereinto. Further, a pressing body pressed by the coil spring is inserted into a hole which is provided in the guide case, and is structured such as to energize the bearing within the guide case in the predetermined preload direction. In this case, the adjusting screw is threadably attached to a lock nut for preventing it from slacking.
In the motor-driven power steering apparatus described in Japanese Patent No. 3646205, each of the parts including the guide case, the adjusting screw, the coil spring, the pressing body and the lock nut constructing the preload means is individually embedded in the fitting hole and the retaining hole of the gear housing, and an assembling characteristic is not good.